Modern switched telephone networks are controlled using a separate signaling network called a Common Channel Signaling (CCS) system to set up and tear down intra-network and inter-network telephone calls. In North America, most switched telephone networks operate using a CCS protocol known as Signaling System Seven (SS7). The SS7 protocol provides detailed procedures for call set-up and tear down. All network functions revolve around and depend upon a strict adherence to those procedures. Auxiliary functions such as toll call billing are dependent upon the signaling protocol prescribed by SS7. It is known, however, that under certain circumstances, signaling procedures are innocently or deliberately not complied with by certain call terminating equipment. This non-compliance can result in a significant loss of revenue for telephone service providers.
For example, it is known that an answer indication is not always returned by equipment connected to a switched telephone network. It is also known that certain switched telephone networks or elements in certain switched telephone networks may fail to provide an answer indication on call completion. This problem is referred to as "no answer supervision" and it appears to account for the greatest source of revenue loss suffered by telephone service providers in North America. For example, it has been determined by actual measurement that in 1997 Bell Canada averaged 300,000 to 400,000 unbilled toll call minutes per day as a result of no answer supervision. It can be extrapolated that in North America, in excess of 3 to 4 million minutes per day are unbilled as a result of no answer supervision.
The equipment most often responsible for no answer indication is the Private Branch Exchange (PBX) connected directly to a switching node of a telephone service provider. PBXs are complex parameter driven pieces of equipment and the lack of answer indication is not necessarily consistent or intended to deceive. For example, a PBX which provides an answer indication during business hours may not do so when switched to "night answer". Since a PBX administrator may not be aware of this problem, it is not likely to be corrected.
Another network element that frequently fails to provide an answer indication on call completion is the Interactive Voice Response (IVR) unit which is increasingly used in the modern Public Switched Telephone Network (PSTN). IVRs were traditionally excluded from a requirement to provide answer indications on the theory that they were only used to provide call information or call redirection. Such units are now used for a variety of other services, however, such as voice mail, order entry, facsimile store and forward, etc. Calls to IVRs now consume tens of thousands of minutes of network resources daily and most of those minutes are not billed.
A further source of lost revenues are calls switched to other switched telephone networks in the PSTN which fail to provide an answer indication on call completion. If no answer indication is provided, the calls cannot be billed or included in inter-network rate settlements, which can likewise lead to significant loss of revenue.
The problem of answer supervision has been addressed for individual pieces of equipment such as an autonomous pay telephone described in U.S. Pat. No. 5,134,651 which issued to Ortiz et al. on Jul. 28, 1992. Ortiz et al. describe a method and apparatus for telephone call answer supervision, capable of detecting the completion of both national and international calls, which employ different answer confirmation formats. The apparatus also provides confirmation that a placed call has been answered if at least one of the following conditions occurs:
1) a tone of given frequency, transmitted on the transmit line of a telephone, is detected and received back on the received line; and PA1 2) audio activity having a non-repetitive cadence is detected on both transmit and receive lines of the telephone. PA1 monitoring call set-up messages exchanged over a common channel signaling system of the switched telephone network to determine whether a call answer message is received from a terminating end of the call before a predetermined criteria respecting the call is satisfied; and PA1 generating at least one common channel signaling message to control the call in an instance when the call answer message is not received before the predetermined criteria respecting the call is satisfied. PA1 monitoring common channel signaling links to detect call control messages; PA1 copying call control messages to a data processing unit which tallies the messages to track the progress of individual calls; PA1 determining at the data processing unit when the progress of an individual toll call fails to conform to an expected call pattern because the switched telephone network does not recognize the call as having been answered; and PA1 acting to control the individual call or acting to ensure that other calls to a same called number as the individual call are routed to ensure that they may be controlled. PA1 means for monitoring at least one common channel signaling link in a common channel signaling system for controlling the switched telephone network to track the progress of individual calls having control messages carried by the at least one common channel signaling link; and PA1 means for generating common channel signaling messages and transmitting the common channel signaling messages into the common channel signaling system to control the progress of calls determined by the means for monitoring to be calls which do not conform to an expected call pattern respecting call answer.
Ortiz et al. also describe an autonomous pay telephone, such as a mobile telephone, which includes an arrangement for determining and accessing a call rate, per unit of time, for a call placed to a given telephone number, a display for displaying, in real-time as the call progresses, charges for a completed call as well as an accounting arrangement for storing charges assessed for each completed call.
Although specialized pieces of equipment such as that described by Ortiz et al. provide answer supervision, there exists a need for a method and a system for providing answer supervision in a switched telephone network so that significant revenues are not lost as the result of equipment or networks which fail to provide an answer indication on call completion.